Studies are in progress on the initiation of DNA replication in E. coli and the structure of replication origins. We are investigating the relationship between the ability of DNA sequences to support ATPase activity of factor Y (an E. coli pre-priming replication protein) and their activity as single-strand DNA replication origins. Studies on DNA replication in yeast are focusing on the replication activity of endogenous 2 micron mini-chromosome in nuclear extracts. This activity is being characterized.